The invention relates to a radio frequency (=RF) power amplifier. Switched mode RF power amplifiers based on delta sigma modulators are e.g. known from M. Iwamoto et al., Electronics Letters, 8th Jun. 2000, Vol. 36, No. 12, page 1010-1012.
Wireless communication systems of the 3rd generation (“3G”) and later use modulation formats which yield RF (radio frequency) output signals featuring high peak to average ratios. The amplification of such signals imposes high requirements on power efficiency and linearity to the transmitter line up, especially for the final stage of the RF power amplifier, where by far the largest share of energy is dissipated.
So called switched amplifiers have a theoretical power efficiency of 100%, going along with high linearity. A switched amplifier is e.g. known from M. Iwamato, see above. M. Iwamato discloses a class S amplifier wherein a bandpass delta-sigma modulator (BPDSM) does a 1-bit A/D (analogue to digital) conversion, generating a fast pulse sequence which is fed into a transistor based switching amplifier. The output signal of the switching amplifier is fed into a bandpass filter for reconstruction purposes.
The physical realization of the switched amplifier concept, however, involves losses and signal distortion e.g. caused by the parasitics of the transistor based switching devices and the reconstruction filter. Examples are parasitic gate capacitance and non-zero on-resistance in the switching field effect transistor, device mismatch between components, and filter losses.
Additionally, the efficiency is often reduced due to limited coding efficiency in case of use of delta-sigma modulators. For the reconstruction filter, in particular in case of a BPDSM based Class-S amplifier, challenging termination requirements in the stop bands and also low insertion loss in the transmission band have to be guaranteed in order to achieve good Class-S performance.
It has been proposed to replace a transistor based switching device with an array of nano-switches, thus avoiding disadvantages which are caused by the analogue nature of a transistor, which has originally been designed as a device for amplification of analogue signals. The nano-switches of such an array are switched identically with the fast pulse sequence of the bandpass delta-sigma modulator. However, a reconstruction filter is still necessary, resulting in distortions and losses, and coding efficiency is still limited by the BPDSM.